Most automatic transmission devices need that clutches be engaged when a rotatory speed exceeds a predetermined threshold, which is often variable depending upon the transmitted torque.
The higher the torque, the higher is the threshold above which a gear set, often embodied as an epicyclic train, has to shift up into direct drive by engagement of the clutch.
Such relatively complicated function is commonly performed by a clutch-engaging jack which is controlled according to information provided by speed sensors and torque sensors.
U.S. Pat. Nos. 2,261,106 and 4,111,291 disclose automatic clutches in which rocking flyweights apply to friction coupling means an engaging thrust which increases with a rotatory speed of a rotatory element of the mechanism.
However, such clutch constructions are not easily practicable because the articulated structure of the flyweights has to resist high centrifugal forces and is time-consuming during manufacture.
The articulated structure needs many flyweight--axles disposed tangentially around the axis of the mechanism. The axles are cumbersome and limit the number of flyweights that can be used in a same mechanism.